In Windows XP or similar, you can have Vim open a chm (HTML Help) file at a specific topic. The following example launches the ASA9 Help file on the CWORD at the cursor, or at the visually-selected topic.

Unfortunately keywordprg requires that the keyword become the last argument to the program. Moreover the keywordprg
is run through vimrun which requires you to close the command prompt that launch the program manually (the "Press any key to close this window" message).